With the development of technologies, the application of WDM is gradually converted from long distance aspect to metropolitan area aspect. In the metropolitan area WDM application, because of the open interface, the WDM can directly provide a bearer channel for a plurality of services, such as asynchronous transmission mode (ATM), IP, and synchronous digital hierarchy (SDH) and so on, and consequently there are frequently a plurality of non-SDH services transmitted directly in the wavelength channel. Furthermore, since the bearing capacity of signal wavelength channel becomes increasingly greater from 2.5 Gbit/s to 10 Gbit/s or 40 Gbit/s, the situation of convergence transmission of services in the signal wavelength channel often happens, i.e., a plurality of services, after being converged, occupy a same wavelength channel to perform the transmission.
In urban area WDM application, in order to improve the whole survival performance of the network, a plurality of optical add-drop multiplexing (OADM) sites are generally combined into a loop network, and the survival performance of the WDM loop network is improved through using a protection mode similar to that of SDH loop network. Furthermore, in order to improve the reliability of the network, a protection and recovery method is generally introduced into the WDM system. At present, the most common protection mode in the WDM loop network comprises a client service 1+1 protection and an optical channel shared protection.
The optical channel shared protection means that two wavelengths on two loops of a two-fiber bidirectional loop network are occupied to realize the shared protection of a plurality of inter-segment services, in which the two wavelengths of internal and external loops functions as run and protection for each other. A plurality of inter-segments of the external loop occupy a same wavelength to transmit services, wherein when any one inter-segment is disconnected, the interrupted service can be switched onto the internal loop shared protection channel. Similarly, the reverted working service occupies another wavelength of the internal loop for transmission, and meanwhile the corresponding wavelength of the external loop is used as the shared protection channel thereof
Generally speaking, the nodes which take part in the optical channel shared protection need to support three functions: service adding, service passing through and service dropping. The service adding means that when the working channel of the local adding service goes wrong, the service can be switched onto the protection channel to be transmitted; the service passing through means that the protection service of other nodes can pass through this node; and the service dropping means that when the working channel of the local dropping service goes wrong, the service can be switched onto the protection channel.
FIG. 1 shows a general structure of the optical channel shared protection. The client service after being accessed from a convergence unit, is output from the line side of the convergence unit, and then is multiplexed onto the line by a multiplexing unit after passing through a protection switching device; and the de-multiplexing direction is contrary thereto. The input and output of the line side of the convergence unit use the standard wavelength specified by ITU-T.
For the node of the service adding, in normal state, the output of the line side of the convergence unit is connected with the adding interface of the wavelength at the present direction via the protection switching device; and in fault state, the output of the line side of the convergence unit is switched to the adding interface of the wavelength at the other direction. For the node of the service passing through, in normal state, there is no switching operation; and in fault state, the protection channel after being de-multiplexed, is directly connected with the adding interface of the protection channel in the other direction by the protection switching device. The node of the service dropping, in normal state, receives the working channel; and in fault state receives the protection channel in the other direction.
The above-mentioned optical channel shared protection uses a pair of wavelengths as a basic unit, wherein firstly the wavelengths are de-multiplexed on the lines at two directions of east and west by an add-drop multiplexing unit, the wavelengths are processed by the protection switching unit, and then the wavelengths are multiplexed onto the line by the add-drop multiplexing unit. In the scheme, when the channel layer or the multiplexing segment layer goes wrong, effective protection can be provided; however when the convergence unit or the adding service goes wrong, because the working channel and the protection channel are both having fault, effective protection can not be provided.
The client side 1+1 protection which is widely used in practical at present can provide protection for client services, however, since it is not a shared protection mode in principle, it also has the disadvantages by itself, that is, a plurality of wavelengths are required to be occupied when a plurality of inter-segment services on the loop network need protection.